J. J. Harris Teall—Cheviot Andesites and Porphyrites. 103 
as to size and form, and there is in both sections the same well- 
marked fluidal structure. In the majority of cases it would not be 
difficult to separate the one series from the other, because fluidal 
structure is usually absent in the pitchstone porphyrites, and the 
felspars of the ground-mass are more fully developed. 
The principal constituents of the rock are (a) large porphyritic 
felspars, (b) pyroxene, (c) felspars of the ground-mass, (d) irregular 
plates of a black opaque mineral (magnetite or ilmenite), (e) glassy 
base containing various devitrification products. Apatite and a hex- 
agonal mineral, probably hematite, occur as accessory constituents. 
Large Felspars.—These measure from 2 to 3 mm. across, and 
seem to be pretty equally developed in the directions of the different 
erystal axes, although lath-shaped forms occasionally occur which 
are decidedly longer than broad. The regularity of the outlines of 
these felspars is frequently interrupted by creeks and inlets of the 
eround-mass, while the more perfect forms often contain a large 
number of inclusions of the same material. In many cases the 
crystals are completely honey-combed by these ramitying inclusions, 
so that any attempt to obtain them for separate analysis would 
probably not be attended with very satisfactory results. A zonal 
arrangement of the larger inclusions is sometimes seen, but on the 
whole one is not. struck with this feature. A band of small inclu- 
sions may frequently be seen close to the edge of well-defined 
crystals. Under crossed Nicols the striations characteristic of twinning 
on the albite type are usually seen, but the lamella frequently show 
a want of persistence and regularity. A second set of lamelle is 
occasionally seen crossing the first at a high angle, and this, I 
presume, may be taken to indicate twinning on the pevicline type. 
Pyroxene.—This occurs in well-defined eight-sided sections, in 
elongated lath-shaped sections, and in irregular crystalline grains 
with more or less rounded angles. In very thin sections it is almost 
colourless; in thicker sections it exhibits under certain circumstances 
faint dichroism. Thus in sections which show more or less parallel 
cleavage cracks a greenish tint may be sometimes observed when 
the cracks are at right angles to the long diameter of the polarising 
Nicol, and a brownish tint when they are parallel to this diameter. 
Separate crystals or grains rarely measure more than 1 mm. in their 
largest diameter, except when they occur as narrow lath-shaped 
sections. The eight-sided sections are doubtless more or less at 
right angles to the principal axis. They show that the prismatic 
cleavages are fairly well developed, and that there is no marked 
pinakoidal cleavage. The pinakoidal faces are largely developed at 
the expense of the prismatic faces, and it is worthy of note that this 
feature is especially characteristic of the augites in augite-andesites." 
Sections more or less parallel to the principal axis show the 
usual parallel cleavage cracks and give extinctions referred to these 
eracks varying from ’ to 44’. Twinning parallel to a pinakoidal 
face (doubtless the ortho-pinakoid) may frequently be recognized. 
These facts appear to establish the existence in the rock of a clino- 
1 Rosenbusch, Mikroskopische Physiographie der massigen Gesteie, p. £10. 
